(1) Field of the Invention
The invention relates to filtering and demodulating passband signals obtained by modulating a carrier in accordance with an information-carrying baseband signal whose band-width is limited to a given maximum frequency.
In the transmission of information by electromagnetic means modulation of a carrier is often used at the transmitter end for obtaining a passband signal which is properly adapted to the properties of the transmission path. At the receiving end the signal derived from the transmission path is demodulated after unwanted signal components originating from the transmission path have been suppressed by means of a bandpass filter (the so-called premodulation filter). Unwanted signal components which are produced in the demodulation process are often suppressed by means of a low-pass filter (the so-called post modulation filter).
The invention results from investigations in the field of a 2400 Baud AM-modem for data signal transmission but is not limited thereto as the same principles can be used for other data rates, for information-carrying signals of another kind and for other modulation methods such as VSB, PSK and QAM. Although this 2400 Baud AM-modem will be discussed hereinafter this should not be interpreted as a limitation of the range of application of the principles according to the invention.
(2) Description of the Prior Art
U.S. Pat. No. 3,753,115 discloses an arrangement for filtering and demodulating passband signals obtained by modulating a carrier in accordance with a baseband signal whose bandwidth is limited to a given maximum frequency, which arrangement is provided with means for filtering the passband signals according to a first and a second passband characteristic for generating first and second filtered passband signals, which passband characteristics, apart from their asymmetrical distortion relative to their central frequency, are versions from one another, shifted 90.degree. in phase, means for demodulating the first and second filtered passband signals with an in-phase carrier and a quadrature carrier respectively for generating first and second demodulated signals, and means for combining the first and the second demodulated signals. In particular, the use of two transversal premodulation filters and two analog modulators eliminates the influence of that distortion in the transfer characteristic of this premodulation filters which is caused by a limitation of the duration of the impulse response of these filters and which is asymmetrical relative to the central frequency of these filters; in addition it accomplishes that no post-modulation filter is required.
The structure of this prior art arrangement is of a hybid nature, that is to say that in the transversal filters use is indeed made of digital delay elements but that otherwise analog circuit components are used, such as resistors, for the weighting networks wherein the filter coefficients are fixed, and analog modulators. A drawback of this hybrid structure is the fact that once such an arrangement is fully integrated in one semiconductor body it is difficult to modify given parameters, such as filter characteristics, so that the arrangement is not programmable. Another drawback of this hybrid structure is the fact that for a proper correction of the influence of the asymmetrical distortion in the transfer characteristic of the premodulation filter high requirements are imposed on the modulators as regards d.c.-offset and other imperfections which are difficult to avoid in analog circuits whereas it is not possible to use digital modulators behind the filters without additional ADC circuits.